Electrochromic display with analog gray scale

ABSTRACT

A display device comprising a plurality of independently addressable pixels ( 1 ). Each pixel comprises: a first substrate ( 6 ); a second substrate ( 7 ); an electrochromic material ( 2 ) disposed between said first substrate ( 6 ) and said second substrate ( 7 ). At least two independent electrodes ( 3, 4 ) are associated with said first substrate ( 6 ). An independent counter-electrode ( 5 ) is associated with said second substrate ( 7 ). Each respective electrode is connected to an independently controllable voltage source. The display device has means for controlling the voltage applied to each respective electrode ( 3, 4, 5 ) for producing non-uniform electric fields in each pixel ( 1 ), for causing partial switching of the electrochromic material ( 2 ) from a first state to a second state to generate an area-ratio-defined pixel gray level.

FIELD OF THE INVENTION

The present patent application relates to the field of electrochromicdisplay devices and particularly to a method and apparatus for providinggray scales for such display devices. More specifically, the presentpatent application relates to an efficient system for providing analoggray scales for electrochromic display devices. The present patentapplication also relates to a computer program product comprisingsoftware code portions for achieving the system and method for providinganalog gray scales for electrochromic display appliances when saidproduct is run on a computer.

BACKGROUND OF THE INVENTION

Electrochromic display devices have recently been studied as candidatesfor electronic paper type displays. However, the slow switching speedand high power consumption of the electrochromic display technologiescommercially available today do not meet the needs of the displaymarket. Of late the trend for improving performance has been the use ofnano-materials such as chemically modified nano-structured mesophorousfilms. Use of such materials has shown promising results. However, oneof the remaining key issues with respect to electrochromic displays isthe generation of gray scales.

One prior art approach to providing gray scales has been to subdivideeach display element (pixel) into a plurality of sub-elements using aplurality of sub-electrodes on one substrate, e.g. providing fifteenelectrodes, splitting each display element into fifteen sub elements,enabling sixteen gray scale levels to be achieved by successiveswitching operation of each of the sub-elements for producing 15 grayscale levels and all sub-elements in their “off” condition providing thesixteenth level.

Yet another prior art approach for providing a gray scale suggests theuse of circular display cells (pixels) having a curved, for exampleconcave, exterior wall of each display cell as well as a plane basewall. By providing the curved wall with an electrode and the plane basewall with a counter-electrode there will be locations with differentelectrode spacing. The circular shape of the electrodes results in aconcentric radial change of the area through which light can betransmitted proportionally to changes in the voltage applied. A systemof this type is disclosed in WO 91 15800.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide animproved apparatus for providing gray scales for an electrochromicdisplay appliance.

This object is achieved by the apparatus according to the invention asspecified in claim 1.

A further object of the invention is to provide an improved method forproviding gray scales for an electrochromic display appliance.

This object is achieved by the method according to the invention asspecified in claim 7.

Further advantageous embodiments of the invention are specified in thedependent claims.

Yet another object of the invention is to provide an improved computerprogram product comprising software code portions for achieving theapparatus and method for providing gray scales for an electrochromicdisplay appliance when said product is run on a computer.

Still other objects and features of the present invention will becomeapparent from the following detailed description considered inconjunction with the accompanying drawings. It is to be understood,however, that the drawings are designed solely for purposes ofillustration and not as a definition of the limits of the invention, forwhich reference should be made to the appended claims. Further it shouldbe understood that the drawings are not necessarily drawn to scale andthat, unless otherwise indicated, they are merely intended toconceptually illustrate the structures and procedures described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, wherein like reference characters denote like elementsthroughout the several views:

FIG. 1 discloses a schematic cross-sectional view of a pixel of adisplay device according to a first embodiment of the invention withelectric field lines shown;

FIG. 2 discloses a schematic cross-sectional view of the pixel of FIG. 1with the entire electrochromic layer in a dark state;

FIG. 3 discloses a schematic cross-section of the pixel of FIG. 1 with acentrally located part of the electrochromic layer in a transparent(bright) state;

FIG. 4 discloses a schematic cross-section of the pixel of FIG. 1 with alarge portion of the electrochromic layer in a dark state;

FIG. 5 discloses a schematic cross-section of the pixel of FIG. 1 withapproximately half the electrochromic layer in a dark state;

FIG. 6 discloses a schematic cross-section of the pixel of FIG. 1 with asmall portion of the electrochromic layer in a dark state; and

FIG. 7 discloses a schematic cross-section of the pixel of FIG. 1 withthe electrochromic layer wholly in a transparent (bright) state.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a schematic cross sectional view of a pixel 1 of anelectrochromic display according to a first embodiment. Each pixel ofthe electrochromic display is independently addressable and can beseparated from each other either electrically or physically, in order toavoid cross-talk between pixels. The pixel 1 comprises: a firstsubstrate 6, which is preferably transparent and made of a material suchas a glass or from a plastic plate; a second substrate 7, which can alsobe transparent in some cases such as for a display lit at the back; anelectrochromic material 2 disposed between said first substrate 6 andsaid second substrate 7; at least two independent conductive electrodes3, 4 associated with said first substrate 6, which electrodes 3, 4 arepreferably transparent; an independent conductive counter-electrode 5associated with said second substrate 7. The pixel 1 also comprises atransparent electrolytic material which is in contact with saidelectrochromic material 2 and said counter-electrode 5. Each respectiveelectrode 3, 4, 5 is connected to an independently controllable voltagesource (not shown). The display device comprises means (not shown) forcontrolling the voltage applied to each respective electrode 3, 4, 5,such as u electronic display control device which can comprise amicroprocessor. In this way, using the display control device,non-uniform electric fields can be produced in each pixel, e.g. asillustrated in FIG. 1 where electric field lines are shown for a casewhere a voltage of approximately 2V is applied to the electrode 3, while0V is applied to electrode 4 and counter-electrode 5. These non-uniformelectric fields will cause partial switching of the electrochromicmaterial 2 from a first (preferably transparent) state to a second state(illustrated by the darker region of the electrochromic material 2). Dueto the non-uniform field distribution, the charge flow will initially beconcentrated to a region close to the positively charged electrode 3. Inconsequence, this region will switch first and the pixel 1 will begenerated with part of its electrochromic material 2 in the dark stateand part of it in the transparent (bright) state. In this manner an arearatio gray level can be generated in the pixel 1. The gray levelgenerated in this way will be defined by the integral amount of chargepassing into the electrochromic material 2 and hence by the time inwhich the electrodes 3, 4, 5 of the pixel 1 are connected to theirrespective voltage sources. The time required to switch to a desiredstate for the preferred type of display is less than 1 second. Erasure,i.e. reset, can easily be achieved through making a change ofpolarities. Such a reset can be used to define a reference state fromwhich all possible gray levels can be generated. If no reset is used, itwill be necessary to remember the previous state of the pixel beforesupplying the correct amount of charge (or discharge) to reach a newgray level. In such a case the electronic display control device willcomprise memory storage means (not shown), where the previouslygenerated gray level is stored and the new gray level to be achieved iscompared with the previous gray level and the required charge(discharge) to be applied to reach the desired gray level is determined.

FIG. 2 illustrates the pixel 1 when 0V is applied to the electrodes 3and 4, while a negative potential is applied to the counter-electrode 5.In this case the pixel 1 will be generated with all of itselectrochromic material 2 in the dark state.

FIG. 3 illustrates the pixel 1 when moderate positive potentials areapplied for a given period of time to the electrodes 3 and 4, while 0Vis applied to the counter-electrode 5. In this case the pixel 1 will begenerated with the regions of its electrochromic material 2 close to thepositively charged electrodes 3 and 4 in the dark state and the part ofits electrochromic material 2 located centrally between these electrodes3, 4 in the transparent (bright) state.

FIG. 4 illustrates the pixel 1 when a slightly higher positivepotential, compared to that of FIG. 3, is applied for the same period oftime to the electrode 3 while 0V is applied to the electrode 4, and 0Vis applied to the counter-electrode 5. In this case the pixel 1 will begenerated with a slightly larger part of its electrochromic material 2closest to the positively charged electrode 3 in the dark state and thepart of its electrochromic material 2 located closest to the 0Velectrode 4 in the transparent (bright) state.

FIG. 5 illustrates the pixel 1 when a positive potential ranging betweenthat of FIG. 3 and FIG. 4, is applied to the electrode 3 while 0V isapplied to the electrode 4, and 0V is applied to the counter-electrode5. In this case the pixel 1 will be generated with approximately half ofits electrochromic material 2 closest to the positively chargedelectrode 3 in the dark state and the half of its electrochromicmaterial 2 located closest to the 0V electrode 4 in the transparent(bright) state.

FIG. 6 illustrates the pixel 1 when a moderate positive potential, asthat of FIG. 2, is applied to the electrode 3 while 0V is applied to theelectrode 4, and 0V is applied to the counter-electrode 5. In this casethe pixel 1 will be generated with a small part of its electrochromicmaterial 2 closest to the positively charged electrode 3 in the darkstate and the rest of its electrochromic material 2 located closest tothe 0V electrode 4 in the transparent (bright) state. The stateillustrated in FIG. 6 essentially corresponds to the state illustratedin FIG. 1.

FIG. 7 illustrates the pixel 1 when 0V is applied to the electrodes 3and 4, and a positive potential is applied to the counter-electrode 5.In this case the pixel 1 will be generated with all of itselectrochromic material 2 in the transparent (bright) state.

As is evident from FIGS. 1 through 7, the use of several independentlycontrollable electrodes in a pixel 1 of a display in accordance with theinvention facilitates the possibility of achieving an analog gray levelin the pixel 1 through controlling the potentials applied to therespective electrodes 3, 4, 5 and the time of application for causing anappropriate part of the electrochromic material 2 to be switched.

When using an electronic display control device comprising amicro-processor, a computer program product comprising software codeportions can be used for controlling the potentials applied inaccordance with the invention for providing gray scales to theelectrochromic display appliance when said computer program product isrun on the micro-processor of the control device.

A method for generating analog gray scales in a pixel 1 of a displaydevice having a first substrate 6, a second substrate 7, anelectrochromic material 2 disposed between said first substrate 6 andsaid second substrate 7, comprises the following steps: providing for atleast two independent electrodes 3, 4 to be associated with said firstsubstrate 6; providing for an independent counter-electrode 5 to beassociated with said second substrate 7; providing for connection ofeach respective electrode 3, 4, 5 to an independently controllablevoltage source; providing means for controlling the voltage applied toeach respective electrode 3, 4, 5 for producing non-uniform electricfields in each pixel 1, for causing partial switching of theelectrochromic material 2 from a first state to a second state forgenerating an area ratio gray level. The method also allows for the stepof providing means for controlling the time during which voltage isapplied to each respective electrode 3, 4, 5. In order to facilitateswitching between gray levels the method also suggests the steps of:providing memory storage means for storing a previously generated graylevel; providing means for comparing a gray level to be achieved with apreviously generated gray level; providing means for determining therequired potential to be applied to each respective electrode in orderto reach a desired gray level.

Thus, while there have been shown and described and pointed outfundamental novel features of the invention as applied to a preferredembodiment thereof, it will be understood that various omissions andsubstitutions and changes in the form and details of the devicesillustrated, and in their operation, may be made by those skilled in theart without departing from the spirit of the invention. For example, itis expressly intended that all combinations of those elements and/ormethod steps which perform substantially the same function insubstantially the same way to achieve the same results are within thescope of the invention. Moreover, it should be recognized thatstructures and/or elements and/or method steps shown and/or described inconnection with any disclosed form or embodiment of the invention may beincorporated in any other form or embodiment be it disclosed, describedor suggested as a general matter of design choice. It is the intention,therefore, to be limited only as indicated by the scope of the claimsappended hereto.

1. A display device comprising a plurality of independently addressablepixels, wherein said pixels comprise: a first substrate; a secondsubstrate; an electrochromic material disposed between said firstsubstrate and said second substrate; at least two independent electrodesassociated with said first substrate; an independent counter-electrodeassociated with said second substrate; wherein each respective electrodeis connected to an independently controllable voltage source; saiddisplay device has means for controlling the voltage applied to eachrespective electrode for producing non-uniform electric fields in eachpixel, for causing partial switching of the electrochromic material froma first state to a second state to generate an area-ratio-defined pixelgray level.
 2. The display device of claim 1, wherein said displaydevice further has means for controlling the time during which voltageis applied to each respective electrode.
 3. The display device of claim1, wherein said display device further has means for controlling thevoltage applied to each respective electrode of the pixel when in thesecond state, to cause a reset from the second state to the first state.4. The display device of claim 1, wherein said display device furtherhas memory storage means for storing a previously generated gray level.5. The display device of claim 4, wherein said display device furtherhas means for comparing a gray level to be achieved with a previouslygenerated gray level.
 6. The display device of claim 5, wherein saiddisplay device further has means for determining the required potentialto be applied to each respective electrode in order to reach a desiredgray level.
 7. A method for generating analog gray scales in a pixel ofa display device having a first substrate, a second substrate and anelectrochromic material disposed between said first substrate and saidsecond substrate, the method comprising the steps of: providing at leasttwo independent electrodes to be associated with said first substrate;providing an independent counter-electrode to be associated with saidsecond substrate; providing a connection of each respective electrode toan independently controllable voltage source; providing means forcontrolling the voltage applied to each respective electrode forproducing non-uniform electric fields in each pixel, for causing partialswitching of the electrochromic material from a first state to a secondstate to generate an area-ratio-defined pixel gray level.
 8. The methodof claim 7, further comprising the step of: providing means forcontrolling the time during which voltage is applied to each respectiveelectrode.
 9. The method of claim 7, further comprising the steps of:providing memory storage means for storing a previously generated graylevel; providing means for comparing a gray level to be achieved with apreviously generated gray level; providing means for determining therequired potential to be applied to each respective electrode in orderto reach a desired gray level.
 10. A computer program product directlyloadable into the internal memory of a digital computer comprisingsoftware code portions for performing the following steps when saidproduct is run on a computer: providing a connection to an independentlycontrollable voltage source for at least two independent electrodes ofan independently addressable pixel of an electrochromic display device;providing control of the voltage applied to each respective electrodefor producing non-uniform electric fields in each pixel; providingcontrol of the time during which voltage is applied to each respectiveelectrode.
 11. A computer program product stored on a computer-readablestorage medium, comprising computer-readable program code means forcausing a computer to perform the following steps: providing aconnection to an independently controllable voltage source for at leasttwo independent electrodes of an independently addressable pixel of anelectrochromic display device; providing control of the voltage appliedto each respective electrode for producing non-uniform electric fieldsin each pixel; providing control of the time during which voltage isapplied to each respective electrode.